Hybrid drive systems for vehicles are known from automotive technology. In the known hybrid drive systems the drivetrain of the vehicle comprises an internal combustion engine which can be coupled by a clutch to an input shaft of a transmission with variable transmission ratios. Furthermore, associated with the input shaft is at least one electric machine, the operation of which can be changed between a generator or as a motor.
Consequently, during motor operation the electric machine can be used to power the vehicle and during generator operation it can be used to charge an energy accumulator. During motor operation, the energy accumulator can supply the energy required for operating the electric machine. For charging the energy accumulator, the electric machine is operated in the generator mode and is then driven by the internal combustion engine.
After very long stationary periods or after a long time in a workshop, the energy accumulators are fully discharged and are no longer ready to operate. To bring them back to a condition of readiness to operate it is therefore necessary to re-charge them. Usually an energy accumulator is re-charged in the workshop using charging equipment. Disadvantageously, this entails having an external source of energy.